In general, products containing printed circuit board (PCB) assemblies must be protected from electrostatic discharge (ESD), which results, in some cases, from a user's natural electrostatic charge build up and consequent discharge with touching of the product housing near an opening. Electrostatic discharge protection is necessary to prevent, or to reduce, damage to the sensitive components on the PCB assembly. Damage resulting from an electrostatic discharge may range from a brief intermittent fault to a catastrophic fault wherein the circuit is no longer capable of functioning. Standards have been set by the International Electrotechnical Commission to ensure proper protection against electrostatic discharge within electronic components. One such standard requires that telephone sets be made to withstand an electrostatic discharge of 15,000 volts. Typically this high-energy ESD is capable of destroying solid-state components in the telephone set.
Many methods of preventing electrostatic discharge from occurring exist in the prior art. One method is to position the telephone set's PCB perimeter far enough in from the walls of the plastic housing of the telephone set so that a 15,000 volt electrostatic discharge at the plastic housing cannot jump from the plastic housing to the printed circuit board. Positioning a PCB in such a manner creates an air gap which by virtue of its length prevents an electrostatic discharge from arcing, up to a given kilovolt level dependent on the size of the air gap. However, positioning a PCB in such a manner is not always possible as an objective in telephone design is to make the set as physically small as possible so that it takes up a minimum amount of space on a desk or table, and so that it is aesthetically pleasing.
Another method is to increase the ESD jump distance, and hence the ESD voltage maximum, by having an internal plastic barricade molded as part of the plastic housing. This forces any ESD to arc a greater distance than it otherwise would and thus prevents an arc from initiating at a given voltage level.
A further method is to put an insulated cover or sleeving over the region of concern which effectively increases the available arc jump distance.
Yet another method is to suspend, or otherwise locate, a wire, soldered to the electrostatic ground at one end, from the PCB much like a lightening arrestor so that it will attract ESD at a particularly sensitive location to prevent it from getting further into the circuitry.
While all of the above methods are effective, they each can add to the complexity of the design of the telephone set or necessitate that the telephone set be made larger to accommodate the electrostatic discharge protection.
Information regarding the state of the prior art may be found in the following references: U.S. Pat. No. 4,821,320 issued Apr. 11, 1989 in the name of T. J. Andert et al.; U.S. Pat. No. 4,667,266 issued May 19, 1987 in the name of Masuoka et al.; U.S. Pat. No. 4,532,419 issued July 30, 1985 in the name of M. Takeda; and U.S. Pat. No. 4,531,176 issued July 23, 1985 in the name of R. L. Beecher.